The present invention relates to a method for producing a beam blank for a universal beam from a flat slab using a breakdown mill incorporated in a universal beam rolling line.
Heretofore, a universal beam was produced by heating a beam blank bloomed from a steel ingot and by rolling it by a universal mill. Recently, however, in view of saving energy and increasing yield rate, a method for rolling the universal beam directly from a continuous-cast slab is becoming widely practised.
Since a continuous-cast slab generally has a less thickness than a steel ingot, it was impossible to form a beam blank having a large flange width from the continuous-cast slab by a conventional caliber rolling process. In order to overcome this disadvantage, accordingly, various new rolling methods have been proposed, some representative examples of which will be briefly described below.
In the first method disclosed by Japanese Patent Public Disclosure No. 70402/80 Official Gazette, a breakdown mill having a number of box calibers with gradually increased groove bottom widths is used to roll a slab with the widthwise direction thereof in the vertical direction, changing the calibers sequentially to reduce the width of the slab into a so-called dog-bone shape and, thereafter, the dog-bone shaped material is rolled into a beam blank of a predetermined shape by a sizing caliber. This method requires a number of box calibers. In general, it is difficult to form a beam blank for a large size universal beam using only a single breakdown mill because of a limitation in length of the roll barrel.
In the second method disclosed by Japanese Patent Public Disclosure No. 41002/81 Official Gazette, a projection is provided at the center of the first box caliber to thereby form a concave groove at the center in the direction of the thickness of the slab, the material is rolled in the next caliber to reduce its width while being held by a similar projection therein so as not to fall down, into a so-called dog-bone shape, thereafter the concavity of the material is eliminated by a box caliber having a normal flat groove bottom, and the material is formed into a beam blank of a predetermined shape by a sizing caliber.
In the third method disclosed by Japanese Patent Public Disclosure No. 5164/80 Official Gazette, a slab is bitten directly by a universal mill and formed into a beam blank of a predetermined shape. In the universal mill, large widthwise reduction is impossible in earlier passes because vertical rolls are generally undriven.
In the fourth method disclosed by U.S. Pat. No. 4,086,801, portions constituting flanges are widened by a plurality of box calibers each having a bulge at the center of the bottom thereof and the apical angles of the bulges being different from each other. In this method, at the time when the material is bitten into the next caliber, the difference in the apical angle between the material and the caliber makes it difficult that the material is bitten from the tip ends of the flange constituting portions and widened equally in both the sides. Accordingly, the material may probably be twisted.
Other incidental steps are, for example, a method disclosed by Japanese Patent Public Disclosure No. 114560/79 Official Gazette, in which a box caliber is used again after a forming caliber, and a method disclosed by Japanese Patent Public Disclosure No. 95402/81 Official Gazette, in which a crowned box caliber is used.
Each of the conventional methods described above utilizes so-called dog-bone deformation in which a slab smaller in thickness than the heretofore used steel ingot is reduced to a large extent in the widthwise direction in such a manner that the reduction force does not effect the central portion of the slab which is expanded only in the ends, into the so-called dog-bone shape. Particularly, in the case of a wide flange universal beam, a total rolling reduction in the width of 500 mm or more is required.
In these prior art methods, the large widthwise reduction produces very large fish-tail in both ends of the beam blank, resulting in a large quantity of crop to be cut off, thereby leading to a decrease in the yield rate of the rolling operation. Further, the necessity for a large number of passes for widthwise reduction sharply reduces the rolling efficiency. The increase in the number of passes causes inevitably a fail in the temperature of the slab and necessitates inclusion of an additional step of cutting the beam blank to the length suitable for charging into a heating furnace and reheating it, between the step of producing the beam blank and the step of making the universal beam from said beam blank.
In the case where a slab having a large flatness ratio is set up with the widthwise direction thereof in the vertical direction and is rolled in the vertical direction, the slab is liable to fall down and to thereby produce a rolling defect which often remains in the product.